1. Field of the Invention
The present invention relates to a door frame structure of a vehicle door, and in particular to a joining structure between an upright pillar sash extending in the vertical direction of the door and an upper sash, which forms the upper edge of the door, the upright pillar sash and the upper sash being butted and joined together by welding.
2. Description of Related Art
A vehicle door frame in which opposed end surfaces of an upright pillar sash (side sash) extending in the vertical direction of the door and an upper sash, which forms the upper edge of the door, are butted and joined together by welding in the door corner portion at the upper end of the upright pillar sash, is known in the art (disclosed in, e.g., Japanese Unexamined Patent Publication 2003-112525). This welding is performed along the periphery of the joined end surfaces of the upright pillar sash and the upper sash to form a butt-weld there along.
The door frame has a tubular portion having a closed cross sectional shape to secure sufficient rigidity, so that an excessive deformation or damage due to high temperatures generated at the time of welding does not easily occur at the tubular portion. On the other hand, if the joined end surfaces of the upright pillar sash and the upper sash in the door corner portion include a thin-walled protrusion or the like, high heat generated at the time of welding increases the risk of burn-through.